1. Field of the Invention
The present invention relates to high power semiconductor devices and, more particularly, to light coupling into power semiconductors.
2. Brief Description of Related Art
Typically, turning ON a power semiconductor device, such as a thyristor by introducing a laser pulse into the blocking region and thereby creating carriers, may shorten the current rise time. Turn-ON in this manner may increase the current rise rate from about 10 kA/μs to almost 1 MA/μs. Generally, light is introduced through an aperture of millimeters in size in the upper metallization layer of the semiconductor device. Variations including the apertures extending through the upper p or n layers have also been tried. When the light is introduced to the semiconductor device in such a manner, the light may not penetrate far under the upper metallization and current may only flow in slow sideways spreading whose speed is typically less than 100 μm/μs. Further, less current flows in the central regions of the apertures since no electrode is present to supply the current. This results in the current flowing in and around the peripheral of the aperture. In one existing method, to obtain a faster current rise time, the light was allowed to enter into the semiconductor device through the side between electrodes. This method allows the light to enter through an edge of the voltage holding region of the semiconductor device, which may be a difficult task to obtain efficient light coupling. Further, when the light enters through the edge, the light penetration is shallow. For example, the light penetration is typically shallow and is around 1 mm with a Nd:YAG-laser in Silicon, and very less with laser diode sources in the 900 s μm range. Therefore, this method limits the current conduction to about 1 mm from the edge of the semiconductor device and requires the semiconductor devices to be in the shape of long (narrow) slivers.
In another existing method, the above mentioned limitation was overcome by introducing leaky fibers into many grooves in the semiconductor device electrode spaced at few millimeters apart. The drawback with this method is that the current conduction took place only around the edges of the grooves and not in the middle of the grooves where no electrode was available to supply the current, or even under the electrode away from the groove edge. Thus a large part of the semiconductor device real estate was not utilized. Further, it is difficult to achieve a fiber leak uniformly over a path more than hundred times longer than that of the fiber diameter, when the diameter of the semiconductor device is measured in centimeters (cm) and the diameter of the fiber is, typically, less than 1 mm.
In general, it is recognized that the amount of light necessary for a fast turn-on of the semiconductor device is the one that will generate about 1017 cm−3 inside the semiconductor device. In a blocking semiconductor device of 1 mm in thickness, this translates into about 1 mJ/cm2 of laser energy per unit device area. For a fast turn-ON, the light has to be introduced within, for example, 10˜100 ns. This in turn translates into light power density per unit area of about 104˜105 W/cm2 on a semiconductor device. The power density carried by a fiber is limited to around 1 GW/cm2 across the cross section area of a fiber. Thus, for the turn-on the light may be expanded from the exit facet of the fiber to the semiconductor area by 4 to 5 orders of magnitude. Whereas, a leaky fiber, can only illuminate a few hundred times its aperture size (about twice of Length/Diameter ratio). This necessitates hundreds or thousands of fibers to couple to a 4-inch semiconductor wafer area.